1. Field of the Invention
The present invention relates to an agricultural implement with a main frame and a movable subframe connected thereto. The agricultural implement has a fluid system which comprises an accumulator which provides a suspension effect to agricultural tools connected to the agricultural implement.
2. Description of the Related Art
US2008/0093093 shows a passive hydraulic system for lowering and raising an agricultural tool. The system has cylinders for moving down and keeping down agricultural tools which are movably connected to a frame. An accumulator which is closable by means of a valve is incorporated in a supply line for hydraulic liquid to the cylinders. When the system is pressurized in a hold-mode, the agricultural tools moving downwards in this case, the accumulator is in flow connection with the supply line by means of an open valve. There is provided a pressure safety valve in order to prevent that the pressure in the accumulator and the cylinders becomes too high. Consequently, when pressurizing the hydraulic system in the hold-mode, the pressure in the accumulator substantially equals a hold pressure in the cylinders. The pressure in the accumulator ensures, together with the pressure in the cylinders, a downward hold force of the agricultural tools. In a lifting-mode of the hydraulic system, the agricultural tools can be brought into a raised position. When the agricultural tools are brought into a raised position, the valve to the accumulator is closed, so that no hydraulic liquid can flow from the accumulator to the reservoir. As a result thereof, the high hold pressure of the accumulator is maintained.
A drawback of this passive hydraulic system is that the agricultural tools have a relatively rigid connection with the frame. As a result thereof, the suspension of the agricultural tools in the lifting-mode during transport leaves room for improvement. Owing to bumps or holes in the pavement, the relatively heavy agricultural tools can move upwards and downwards and generate large forces on the frame. This may lead to damage by cracks in the frame as a result of material fatigue.
U.S. Pat. No. 6,382,326 describes such a hydraulic system with suspension in a transport position by means of an accumulator. The hydraulic system has electro-hydraulically controlled lifting cylinders to move a frame portion upwardly and downwardly in a hinging manner. The lifting cylinders are controllable by associated electro-hydraulic control valves. There is provided a control unit ICU to actuate the control valves on the basis of signals from different potentiometers. In addition to the control unit ICU, there is provided an operable control unit OCU to adjust inlet parameters. On the basis of a comparison between the signals of the potentiometers and the inlet parameters, the lifting cylinders are actuated. By opening the control valves the lifting cylinders can be brought into a lifting-mode. When the lifting cylinders for transport have been brought into the lifting-mode, a suspension system present in the hydraulic system is activated. The suspension system has accumulators which can be brought into open flow connection with the lifting cylinders by operating electric valves. As a result of the open flow connection, the frame portion is resiliently suspended in the transport position.
A disadvantage of the known hydraulic system is that its functioning requires an active control. As a result thereof, the system has a complex configuration with numerous components. The presence of relatively many electronic components makes the system sensitive to operational failure in comparison with a passive system and, moreover, expensive.